These materials are also characterized by a very low hardness, an elastic behaviour, that is to say that they regain their initial shape after a deformation, even of large amplitude, and a softening temperature beyond which they become plastic, which facilitates their processing.
By definition, and by convention for clear understanding of the present description, a product is considered to have a very low hardness when, in the crosslinked and finished state, its hardness is less than 10 Shore A measured according to the ASTM D-2240 (1986) standard, is considered to be very elastic when, in the crosslinked and finished state, its elongation at break, measured according to the ASTM D-412 (1998) standard, is greater than 500%, and is considered to be highly tacky when, in the uncrosslinked state, the distance travelled on a ball tack tester is less than 5 cm. The self-sealing solutions possess these three characteristics.
The test for measuring the tackiness using a ball tack tester, the description of which is given in the present description, in reference to FIG. 3, is not the subject of a standardized standard. This measurement consists in measuring the free length L travelled over the surface of the material to be measured by a ball B endowed with a given initial speed. The tack tester is formed by an inclined plane making an angle of 13°+/−0.08° relative to the horizontal plane, covered with a smooth material of the Mylar® type. The calibrated, 100CR6 smooth ball B made of grade 40 steel has a mass of 80.2+/−0.2 g and a diameter of 26.98+/−0.05 mm. The material M to be measured forms a layer of constant thickness. This material is positioned on a horizontal flat surface adjusted using a spirit level, and the inclined plane is placed on the surface of the material to be measured so as to leave a sufficient length of the surface of said material free. The ball, previously cleaned with ethyl alcohol using a dry cloth, is placed on the inclined plane so that the height H between the plane forming the surface of the material to be measured and the horizontal plane tangent to the upper pole of the ball is equal to 63.5+/−0.5 mm. After having been released, the ball picks up momentum over the inclined plane and rolls freely a length L over the flat surface of the product to be measured. The measurement is carried out under ambient temperature Ta and humidity RH conditions (Ta=23° C.+/−2° C., RH=50%+/−10%).
Such products may have, as matrix, thermoplastic elastomers (TPEs) and in particular stirene thermoplastic (TPS) elastomers, such as stirene/butadiene (SB), stirene/isoprene (SI), stirene/isobutylene (SIB), stirene/ethylene/propylene (SEP), stirene/ethylene/butylene (SEB) or else stirene/butadiene/stirene (SBS), stirene/isoprene/stirene (SIS), stirene/butadiene/isoprene/stirene (SBIS), stirene/isobutylene/stirene (SIBS), stirene/ethylene/butylene/stirene (SEBS), stirene/ethylene/propylene/stirene (SEPS), stirene/ethylene/ethylene/propylene/stirene (SEEPS) block copolymers, and mixtures of these copolymers, as described, for example, in applications WO 2008/080557 and WO 2009/059709 which relate to self-sealing compositions that can be used in pneumatic tires.
They may also have, as matrix, diene elastomers, especially selected from the group consisting of polybutadienes, natural rubber, synthetic polyisoprenes, butadiene copolymers, isoprene copolymers and mixtures of these elastomers, as described, for example, in applications WO 2010/009849 and WO 2010/009851 which also relate to self-sealing compositions for pneumatic tires.
In order to obtain a very low hardness, they may comprise extender oils in a high proportion, in particular for the products in which the elastomer matrix is a thermoplastic. In the latter case, the extender oil is introduced in a proportion of 200 phr (per hundred of elastomer by weight) or more.
However, the storage, handling and preparation of these products, with a view to their conversion, comprises specific difficulties. This is because it is necessary to produce intermediate products that are in the form of continuous strips, the cross section of which is suitable for being able to be used as a product incorporated in processing and conversion processes. And it is impossible to produce such strips without particular precautions, at the risk of having to confront all the drawbacks linked to the high tack and to the deformability of these self-sealing products.
The expression “continuous strip” is understood to mean a strip in which the length is several orders of magnitude greater than the ratio between the cross section of the strip and said length. By way of indication, it is considered that a continuous strip is being dealt with when the length of the strip is a hundred times greater than the ratio between the cross section of the strip and its length.
It is known from the prior art to package tacky products in a film that covers the entire surface of the product, in order to avoid the agglomeration of said products with their supports, or with the adjacent products, during the industrial processing phases.
Conventionally, these films are chosen so that their formulation is compatible with the self-sealing product, which exempts the user from the need to remove the film during the conversion of the product. More specifically, it is arranged so that the film melts and mixes completely with the self-sealing product during the use of said product at a temperature generally above the temperature of the film.
A film in a polymer is therefore chosen, the composition of which is compatible with the formulation of the self-sealing products described above. Products are considered to be physically compatible when the glass transition temperature (Tgm) of the mixture has a single maximum. The glass transition temperature is determined by differential scanning calorimetry (or DSC). By denoting as Tgp the glass transition temperature of the self-sealing product and as Tgf the glass transition temperature of the film, it is observed that the mixture of the two components has a glass transition temperature Tgm between Tgp and Tgf according to the Fox equation, such that 1/Tgm=a/Tgp+(1−a)/Tgf (in which Tg is expressed in Kelvin), and that the glass transition temperatures of the self-sealing product Tgp and that of the film Tgf cannot be detected in said mixture.
The packaging films that are particularly well suited to this type of application are films predominantly based on ethylene vinyl acetate (EVA) and more particularly films of this family such as the films predominantly based on polyolefin elastomer (POE) or on polyolefin plastomer (POP).
The problem posed by the use of packaging films of any nature lies in the fact that these materials, when they are present above a certain proportion, substantially modify the properties of the thermoplastic products that they are used to package, such as, for example, the modulus of elasticity or the elongatability. Therefore, particular attention is necessary when it is desired to deposit a protective film on the surface of these materials.
For the products and films considered, the acceptable dilution ratios must be less than 1%, and are, as a general rule, less than 0.5% by volume and, preferably, less than 0.3%.
Therefore, to reduce the dilution ratio, it is sought to reduce the thickness of the films used for the packaging of these self-sealing products as much as possible.
To satisfy this requirement and to be free of the difficulties linked, on the one hand, to the non-availability of films of very small thickness on the market and, on the other hand, to the difficulty of using films of very small thickness that have a very low mechanical strength, one solution consists in adapting the cross section of the continuous strip so that the volume ratio between the sealing product and the film remains contained within the acceptable ratios defined above.
It results therefrom that the ratio between the cross section of the strip and its circumference must be greater than a given magnitude, proportional to the thickness of the strip. Thus, for a given film thickness, the more it is sought to reduce the dilution ratio, the more the cross section of the strip will have to be increased. In practice, it is arranged so that the ratio between the cross section of the strip and its perimeter is greater than or equal to 100 times the thickness of the film, and preferably greater than or equal to 330 times the thickness of the film.
This situation has the effect, for a given film thickness, of imposing the cross section of the strip of self-sealing product. It is nevertheless observed that this technical choice may pose problems of control of the thermodynamics during the extrusion of these strips, but may also render the strip difficult to use in downstream conversion processes due to its excessively large size.